f2fs: add missing doc for fault injection sysfs

We supported configuring fault injection parameter via sysfs w/
below commits, however, we forgot to add doc entry, fix it.

commit 08796897 ("f2fs: add fault injection to sysfs")
/sys/fs/f2fs/fault_injection/fault_*

commit 1ecc0c5c ("f2fs: support configuring fault injection per superblock")
/sys/fs/f2fs/<device>/fault_*

Cc: Sheng Yong <shengyong@oppo.com>
Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>